The invention relates to a PTFE article comprising microporous PTFE, for example stretched polytetrafluoroethylene (ePTFE), with a filler, and to a process for the production of a PTFE article of this type.
Polytetrafluoroethylene (PTFE) has been used for a variety of purposes owing to its special properties. For certain applications, PTFE is provided with a filler in order to modify the properties of the PTFE for the particular application. For example, U.S. Pat. No. 4,949,284 (Arthur et al) discloses introducing a ceramic filler (SiO.sub.2) and a small amount of glass microfibers with a particle size of 10-15 .mu.m into PTFE material. This material is used for the production of printed wiring boards (PWBs).
The ceramic filler improves the dimensional stability compared with PTFE/glass fiber composites. In general, these substances are nonporous.
A special way of processing PTFE provides a porous material. Paste extrusion of PTFE material and stretching of the extruded material at a temperature of up to about 330.degree. C. followed by sintering above the crystalline melting point of the PTFE material provides a porous PTFE structure composed of nodes connected by fibrils. This PTFE is expanded by stretching and will be referred to as ePTFE. The stretching is carried out, for example, at a rate of 10%/second. The extent to which the material is stretched determines its density (specific gravity).
Upon paste extrusion and stretching an ePTFE membrane is obtained which is permeable to air and water vapor, but impermeable to liquid water. The material can be shaped and then used, in sintered or unsintered form, as a sealant material. The material can be laminated, i.e. provided on one or both sides with one or more layers, to give multilayer materials. Numerous applications of such materials are known from the prior art.
For specific applications, it is known to fill such porous PTFE (ePTFE) with a filler in order to achieve certain mechanical, physical, chemical or electrical properties. EP-A-0 463 106 (Mortimer) discloses a film and the production thereof by mixing .about.PTFE with a filler. The fillers given are, for example, aluminium oxide, titanium dioxide, glass fibers, carbon black, activated charcoal and the like. Filler particles have a size between 1 and 100 .mu.m. In general, the proportion of the filler is greater than 25% by vol.
EP-A-0 463 106 (U.S. Pat. No. 4,985,290) discloses more specifically a filled PTFE film having a thickness of between 2.5 and 127 .mu.m, where the filler has a size of less than 40 .mu.m, preferably 1-15 .mu.m. During the production process, the film formed by paste extrusion and/or calendering is stretched and then compressed to the desired thickness. The compression not only achieves the desired film thickness and reduces the porosity, but also serves to fix the fillers. The film can of course not be thinner than the diameter of the ceramic filler particles. If the film is to be 3 .mu.m thick, virtually no filler particles may be larger than 3 .mu.m in diameter.
Although, for example, the electrical properties of expanded porous PTFE (ePTFE) can be affected by addition of carbon/metal particles, the original mechanical properties of this porous PTFE suffer to the extent to which the proportion of filler is increased. The filler particles represent defects in the abovementioned node/fibril structure which modify the properties of the unfilled ePTFE.
EP-A-0 443 400 (Arthur) discloses introducing inorganic particles into unstretched (nonporous) PTFE in order, for example, to reduce the dielectric constant. The filler particles may be coated.
U.S. Pat. No.4,153,661 (Ree Buren et al.) discloses a PTFE article containing a filler, for example titanium oxide; the particle size given is in the range from 100 nanometers to 600 .mu.m, preferably from 1 .mu.m to 100 .mu.m. Firstly, PTFE and filler are mixed and processed further to produce fibrils in the material. The material is then subjected to biaxial calendering. A PTFE material of this type is unstretched and cannot be stretched further after said processing steps. In order to achieve fibrillation, a relatively large particle size is desirable. Examples given are ranges from 20 to 30 .mu.m and from 200 to 250 .mu.m. If a membrane is produced from this material, the minimum thickness of the membrane is restricted to 20 .mu.m by a filler having such a particle size. U.S. Pat. No. 4,194,040 discloses a film material produced using, as starting material, particles having a mean diameter of less than 10 .mu.m. The material contains 1-15% of PTFE as matrix for fixing.
For coating substrates and the like, it is known to prepare, by polymerization of aqueous microemulsions, a latex containing particles of organic polymer with side chains having a mean particle size of between 0.01 and 0.5 .mu.m (U.S. Pat. Nos. 5,376,441; 5,385,694 and 5,460,872, Wu et al.). The latex material can serve to cover the walls of porous substrates; the latex material is not bound into the structure.
For example, EP-A-0 437 721 (Tamaru et al.) discloses a semi-sintered PTFE multilayer structure containing an additive in at least one of the layers. The particle size of the additive is between 0.03 and 20 .mu.m , preferably between 1 and 10 .mu.m. An express warning is given against the size being below a minimum value: "If it (the particle diameter) is less than 0.03 .mu.m, the action of the additive is inadequate, and if it is greater than 20 .mu.m, the molding properties of the PTFE fine powder are impaired". The preferred particle size here is thus clearly above 1000 nm, apparently because a noticeable action of the filler is not expected in the case of smaller particles.
According to EP-A-0 437 721, the additive is not involved in fiber formation ("non-fiber-forming material"). This contradicts the teaching of the present invention, according to which the nanoparticles are part of the structure. According to this publication, additives having a particle size in the manometer range cannot modify the properties, for example the mechanical properties, of the material. Surprisingly, however, the inventors found that it is precisely the use of extremely small particles as filler for stretched PTFE which has a special action.